Recently, ceramic honeycomb filters comprising ceramic honeycomb structures with a plurality of flow paths sealed alternately at both ends have been finding applications in removing particulates containing carbon as a main component from exhaust gases from diesel engines. As shown in FIGS. 3 and 4, a porous ceramic honeycomb filter 1, which is usually substantially cylindrical or elliptical, comprises (a) a porous ceramic honeycomb structure 10 comprising an outer wall 20, and porous partition walls 30 disposed inside the outer wall 20, with a large number of flow paths 40 encircled by the outer wall 20 and the porous partition walls 30 or by the adjacent porous partition walls 30, and (b) plugs 50, 52 for alternately sealing the inlet-side opening ends 12 and the outlet-side opening ends 13 of the flow paths 40.
An exhaust gas-cleaning mechanism in the honeycomb filter 1 is shown in FIG. 4. Because the plugs 52 are disposed at the outlet-side opening ends 13 of the flow paths 40, an exhaust gas (shown by the black arrow 90) flowing into flow paths 41 open at the inlet-side opening end 12 of the honeycomb filter 1 passes through the pores of the partition walls 30, and is discharged from flow paths 43 open at the adjacent outlet-side opening end 13 (shown by the white arrow 92). At this time, particulates contained in the exhaust gas are captured in the pores of the partition walls 30, resulting in the cleaning of the exhaust gas. When particulates captured in the pores exceed a predetermined amount, the clogging of the pores takes place, causing the pressure loss of the honeycomb filter 1 to increase and thus resulting in decrease in an engine output.
When the pores are clogged, the supply of an exhaust gas to the honeycomb filter 1 is stopped, and the captured particulates are burned by a burner or an electric heater to regenerate the honeycomb filter 1. In a case where particulates are burned and removed by a burner or an electric heater, the larger the amount of particulates captured, the more difficult it is to uniformly control the temperature in the honeycomb filter 1. Particularly in portions in which particulates are accumulated in a high concentration, the temperature of the honeycomb filter 1 is likely elevated, the honeycomb filter 1 is highly likely to be broken by thermal stress generated by the burning of particulates. In some cases, the temperature of the honeycomb filter 1 is elevated to the melting point of a ceramic material forming the partition walls 30 or higher, so that the partition walls 30 are broken or melted. On the other hand, if the highest temperature of the honeycomb filter 1 were suppressed to avoid breakage and melting, the regenerated honeycomb filter 1 would fail to have sufficiently low pressure loss because of the cinders of particulates.
JP 3-68210 B discloses an exhaust gas-cleaning structure for easily regenerating a honeycomb filter, comprising a honeycomb structure having a large number of cells, plugs for alternately sealing the inlet and outlet sides of each cell, a shell covering the honeycomb structure and having exhaust gas inlet and outlet, and a heating means disposed on the inlet side of the honeycomb structure in the shell, the plugs on the inlet side being disposed inside the inlet-side opening ends of the cells, and the plugs on the outlet side being disposed at the outlet-side opening ends of the cells. This exhaust gas-cleaning structure has a space between the inlet-side plugs and the outlet-side opening ends of the cells, and a large amount of particulates are attached to the partition walls of the cells facing the space. Accordingly, heat generated by the heating means disposed at the inlet-side cell opening ends is effectively conducted downstream, making it easy to burn particulates in a downstream region.
However, in the honeycomb filter of JP 3-68210 B, because the heating means is disposed only on the inlet side of the cells, it is difficult to uniformly control the temperature inside the honeycomb filter long in a flow path direction from the inlet side to the outlet side. Accordingly, as the amount of particulates increases, the temperature locally becomes too high due to heat generation by the burning of particulates, making it likely that the honeycomb structure is broken and melted. Also, the control of the heating means should be conducted precisely, resulting in a high energy cost, which in turn makes the overall exhaust gas-cleaning apparatus expensive.
JP 7-106290 B discloses a filter for particulates in a diesel exhaust gas, which comprises a catalyst comprising a platinum-group metal and an alkaline earth metal oxide, which is carried on the surfaces of partition walls, the burning start temperature of particulates being lowered by the action of the catalyst to continuously remove the particulates. This filter can be continuously regenerated even at such low temperatures as the exhaust gas temperatures of diesel engines, thereby preventing the clogging of the filter by particulates.
However, this filter cannot prevent pressure loss increase due to clogging by particulates in some cases. The reason therefor is that because driving continues at an exhaust gas temperature lower than about 300° C., which is the lower limit of the activation temperature of a catalyst carried on the filter, in a low-speed driving environment like a big city, the burning of particulates by the catalyst is less likely to be conducted well.
To solve such problems, JP 2002-122015 A discloses a method for cleaning an exhaust gas by capturing particulates in an exhaust gas by a catalyst-regenerable filter disposed in the middle of an exhaust pipe in which an exhaust gas from a diesel engine flows and burning and removing particulates accumulated in the filter, comprising injecting a fuel inside a filter region upstream of the plugs at the start of the diesel engine; igniting the fuel to elevate the temperature in the filter to a temperature substantially equal to or higher than the lowest activation temperature of the catalyst; and injecting a fuel into a filter region upstream of the plugs without ignition during the subsequent stationary operation, thereby causing heat generation by an oxidation reaction of the fuel on the catalyst, to maintain the temperature in the filter to a temperature substantially equal to or higher than the lowest activation temperature of the catalyst. The catalyst is always kept in a stably active state by the oxidation reaction of the fuel without regard to the operation conditions of the diesel engine, so that particulates captured in the filter are continuously burned. However, even this exhaust gas-cleaning method is likely to fail to prevent the filter from prematurely suffering from pressure loss increase by clogging by particulates.